Optical systems that produce high-power optical output signals such as optical-fiber based lasers and power amplifiers can suffer damage if a backward-traveling optical signal (e.g., from a reflection of a forward-traveling high-power optical output signal after it has left the power-amplification stage of the laser system) re-enters the power-amplification stage where it can get amplified and then damage components in the laser system. Thus, there is a need in the art for technology and methods that can detect, prevent, and/or mitigate problematic backward-traveling beams, especially in fiber-based optical amplification systems.
Further, spectral beam combining (SBC) of beams from fiber lasers is a promising technology enabling a very-high-power laser system with excellent beam quality. An efficient fiber laser type for such systems can be the ytterbium-doped (Yb) fiber laser, which lases around 1,060 nm. If the output beams from a plurality of such fiber lasers are spectral-beam combined, the resulting optical beam can have extraordinarily high power. There can be a need to detect, prevent, and/or mitigate problematic backward-traveling beams (e.g., from reflections or stimulated Brillouin scattering (SBS)) in SBC systems.
Even when a fiber amplifier or fiber laser is designed to compensate for the above effects, there will be a limit on the maximum power that can be obtained from a single fiber when scaling to larger fiber sizes and/or lengths, pump powers, and the like.
U.S. Pat. No. 6,192,062 to Sanchez-Rubio et al. titled “Beam combining of diode laser array elements for high brightness and power” and U.S. Pat. No. 6,208,679 to Sanchez-Rubio et al. titled “High-power multi-wavelength external cavity laser” describe the fundamental techniques of spectral beam combining, and both are incorporated herein by reference.
In some embodiments of the present invention, the gratings used for spectral-beam combining are “blazed,” i.e., formed with V-grooves having sidewall angles that are asymmetrical with respect to a vector normal to the overall surface of the grating. U.S. Pat. No. 3,728,117 to Heidenhain et al. titled “Optical Diffraction Grid” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves. U.S. Pat. No. 4,895,790 to Swanson et al. titled “High-efficiency, multilevel, diffractive optical elements” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves using binary photolithography to create stepped profiles. U.S. Pat. No. 6,097,863, titled “Diffraction Grating with Reduced Polarization Sensitivity” issued Aug. 1, 2000, to Chowdhury (incorporated herein by reference) describes a reflective diffraction grating with reduced polarization sensitivity for dispersing the signals. The Chowdhury grating includes facets that are oriented for reducing efficiency variations within a transmission bandwidth and that are shaped for reducing differences between the diffraction efficiencies in two orthogonal directions of differentiation. U.S. Pat. No. 4,313,648 titled “Patterned Multi-Layer Structure and Manufacturing Method” issued Feb. 2, 1982, to Yano et al. (incorporated herein by reference) describes a manufacturing method for a patterned (striped) multi-layer article.
U.S. Pat. No. 6,822,796 to Takada et al. titled “Diffractive optical element” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves with dielectric coatings. U.S. Pat. No. 6,958,859 to Hoose et al. titled “Grating device with high diffraction efficiency” (incorporated herein by reference) describes a method for making blazed gratings having dielectric coatings.
U.S. Pat. No. 5,907,436 titled “Multilayer dielectric diffraction gratings” issued May 25, 1999, to Perry et al., and is incorporated herein by reference. This patent describes the design and fabrication of dielectric grating structures with high diffraction efficiency. The gratings have a multilayer structure of alternating index dielectric materials, with a grating structure on top of the multilayer, and obtain a diffraction grating of adjustable efficiency, and variable optical bandwidth.
U.S. Pat. No. 6,212,310 titled “High power fiber gain media system achieved through power scaling via multiplexing” issued Apr. 3, 2001, to Waarts et al., and is incorporated herein by reference. This patent describes certain methods of power scaling by multiplexing multiple fiber gain sources with different wavelengths; pulsing or polarization modes of operation is achieved through multiplex combining of the multiple fiber gain sources to provide high power outputs, such as ranging from tens of watts to hundreds of watts, provided on a single mode or multimode fiber.
U.S. Pat. No. 7,586,671 to Eiselt issued Sep. 8, 2009, titled “Apparatus and method for Raman gain control” and is incorporated herein by reference. The Eiselt patent pertains to optical fiber transmission systems, and optical transport systems employing Raman optical amplifiers, and describes an apparatus and method to control the Raman gain based upon power measurements at one end of the transmission fiber.
There is a need for a method and for an optical device that “taps” (obtains at least a portion of) both a forward-propagating optical output beam from a laser system as well as a backward-propagating optical beam (such as from a reflection), and, based on automatic analyses of the forward and backward portion(s), controls at least one aspect of the operation of the laser system.